An example known of starters of the type having planetary reduction gears built therein is shown in FIG. 1. In this figure, 101 designates an armture, and 102 an armature rotation shaft with a commutator 103 engagingly set thereon at the rear part thereof. To this commutator 103, armature coils 104 are connected.
Numeral 105 designates a brush and holder to be in contact with the commutator 103, which are secured to a rear bracket 107. Numeral 108 designates a bearing, and 109 a yoke of dc motor, which has a plurality of permanent magnets 109a securely set on the internal circumferential surface thereof.
On an end surface of this yoke 109, a front bracket 111 wherein an internal gear 110, which comprises a planetary reduction gear, is engagingly mounted as shown in the figure. With both spur gear 112 and the internal gear 110, a plurality of planetary gears 113 are engaged. Numeral 114 designates a bearing supported by a supporting pin 115 and engagingly set on the internal circumferential surface of the planetary gear 113, and numeral 116 designates a flange for fixedly setting this supporting pin 115, which composes an arm of the planetary reduction gear, with a rotation output shaft 117 fixedly set thereon at the rear. Numeral 118 designates a sleeve bearing, and 119 also a sleeve bearing engagingly fit in the rear inner circumferential cavity of the rotation output shaft 117, which bears the front end part of the afore-mentioned armature rotation shaft 102. Numeral 120 designates a steel ball for giving an taking the thrust force, and 121 a helical spline formed on the outer circumference of the intermediate part of the rotation output shaft 117, with an overrunning clutch 122 in front and in a rear slidably engaged thereon in spline way. Numeral 123 designates a stopper provided at the front end part of the output shaft 117 for controlling the axial shift of a pinion 124, and 125 a sleeve bearing for bearing the rotation output shaft 117 at its front end part, which is engagingly set on the front end inside surface of the front bracket 111. Numeral 126 designates a lever having a rotation shaft 126a, each end of which is to be engagingly fit on the outer peripheral parts of a plunger 128 of an electromagnetic switch 127 and the overrunning clutch 122, as shown in this figure. Numeral 129 designates a movable contact, which is mounted on a rod 131 through an insulator 130, and the rod 131 is in front and in rear movably inserted in a core 132. Numeral 133 designates a fixed contact which is fixed, by means of a nut 134, to a cap 135 which is of an insulator. Numeral 136 designates an exciting coil for urging the plunger 128, which is wound on a bobbin 137 and contained in a case 138 in that state. Numeral 139 designates a lead wire, which connects the fixed contact 133 with a brush of the brush-and-holder 105.
In the following, the operation of this starter is described. By closing a key switch not shown in this figure, an exciting coil 136 of the electromagnetic switch 127 is energized, whereby the plunger 128 is urged to move rearward, to push the rod 131 rearward, thereby setting the movable contact 129 to abut the fixed contact 133. Thereby, power is supplied to the armature coils 104 from the fixed contacts 133 through lead wires 139 and the brush-and-holder 105, causing the armature 101 to produce its turning force. The rotation of the armature 101 is transmitted from the spur gear 112 to the planetary gear 113 and, then, to the overrunning clutch 122, with the speed reduced by the planetary reduction gear. At this time, the pinion 124 which is engaged with the overrunniing clutch 122 is rotationally driven together with the rotation output shaft 117.
On the other hand, the force of the plunger 28 which has been urged as hereabove described rotates the lever 126 counterclockwise with the rotation shaft 126a as the center of rotation, causing the overrunning clutch 122 to slide forward along the axial line together with the pinion 124. Thereby, the pinion 124 engages with, for example, a ring-gear circumferentially provided on a flywheel mounted on a crank shaft not shown in this figure of an engine, thereby to start the engine.
With the conventional engine starter composed as hereabove described, the electromagnetic switch and the DC motor are parallel-axially composed. Therefore, when the conventional engine starter is mounted on the vehicle, the space for the electromagnetic switch becomes necessary on the vehicle side, exclusive of the engine or the engine section, thus limiting engine design layout in vehicles, etc., and involving other problems.
In order to avert such a problem, it was proposed to provide a starter unit in such a simple form as a mere oblong cylinder, with the electromagnetic switch arranged on one axial end side of the DC motor. According to this proposal, the basic composition is such that the plunger rod of the electromagnetic switch unit or the like push rod is extended to the rotation output shaft through the internal passage of the armature rotation shaft; such type of the starter unit is called a coaxial type starter unit, because the armature rotation shaft of the DC motor and the rod of the electromagnetic switch unit are arranged on a coaxial line.
However, when the coaxial type is adopted as proposed, its whole shape would become a simple oblong cylinder, resulting in that its total length becomes long.
In a coaxial type starter with the electromagnetic switch disposed at the rear of the motor, it is necessary that the armature rotation shaft should be hollowed and the rod of the magnetic switch be inserted into the shaft, to push the rotation output shaft. In such case, however, there is a fear that dust or oil invades through the armature rotation shaft or brush powders invasion is likely; and thus there was a problem because complete sealing of the contact chamber of the electromagnetic switch is impractical.
For this reason, the conventional coaxial type starter is so composed as to have its contacts installed in separate places, thereby resulting in a lengthened dimension of the starter; thus, the problem in engine layout has not been solved.
In the above-described coaxial type starter, a coil spring for restoring the aforementioned rotation output shaft to its former position after the engine has started is necessary, raising a problem that, depending on where this coil spring is installed, the total length of the starter may become too long, and the additional problem that the composition of the stopper for restricting the forward movement of the aforementioned rotation output shaft becomes complex.
Further, to avoid interference of the end edge part of the front bracket with, for example, the flywheel inside the engine transmission mission housing, the configuration of the flywheel is too restricted.
Furthermore, heretofore, DC motors employed in, for example, vehicle starter units, etc., have been composed as shown in FIG. 2. Thus the conventional DC motor is equipped with an armature core 101 mounted on a rotation shaft 102 and a commutator 103 supported on this rotation shaft on one side of said armature core 101. This commutator 103 is a so-called face type, having its contacting surface 103a with brushes 105a of the brush unit 105 formed as a perpendicular surface crossing at a right angle to the axial line of the rotation shaft 1, and said commutator 103 is composed of a large number of commutator segments 103b insulated from each other with a synthetic resin. To each of these segments 103b, the terminal part 104a of each armature coil drawn out of the armature core 101 is connected. Accordingly, these terminal parts 104a are closely arranged around the rotation shaft 102 in its circumferential direction, thereby forming a ringed terminal wire group 104.
On the outer circumference of this ringed terminal wire group 104, a fastening ring 200, called a "bind" is provided for preventing outward projection of said terminal wire group 104 due to the centrifugal force, when it is turned together with the armature core 101 and the commutator 103. This bind 200 is generally formed by winding piano wire 200a on the upper surface of an insulating paper 201. In place of the piano wire 200a, a ringed iron plate or a tape impregnated with a resin, etc., may be used.
In FIG. 2, 105b designates a holder for the brush unit 105 and 202 a mold part for supporting the commutator 103 on the rotation shaft 102 through an inset 203.
When a DC motor is utilized as a vehicle starter, sometimes the revolution of the rotation shaft is transmitted to the overrunning clutch through a reduction gear. In a starter of this construction, sometimes the armature core is reduced in size by raising the reduction ratio of the reduction gear for miniaturization and reduction in weight of the DC motor. In this case also, the commutator size is physically determined, so that it cannot be reduced in size in correspondence with the armature core. Accordingly, the commutator becomes relatively larger than the armature core, and when the armature core is reduced at size, its revolution will be in a higher speed. On this account, a very large centrifugal force is imposed on the commutator.
In its normal use, not limited to its utilization for vehicle starters, the commutator, particularly, segments are made larger than the armature core for improving the rectifying condition. When the commutator is made large, brush sweep area is increased and current density is reduced, whereby the rectifying condition is improved and the brushes' service life is prolonged. In such a state also, the commutator will receive a larger force from the centrifugal force, since the commutator becomes relatively larger than the armature core.
Further, in whichever case described hereabove, when the commutator is of the so-called face type in which its contacting surface with brushes is formed in a perpendicular surface which crosses with right angles to the axial line of the rotation shaft, the size of the commutator increases in the radial direction, resulting in larger centrifugal forces. The centrifugal force exerted on the commutator may become larger enough to break the connection of the commutator's segments with an insulator causing segments to burst radially
Accordingly, it is an object of this invention, to solve such conventional problems, and to provide a small engine starter by retrenching its whole length as much as possible.
It is another object to obtain a starter wherein contacts are not soiled by the brush powders from the motor part or by the dust infiltrating through the hollow shaft of the armature.
Further, it is also another object to offer a coaxial type starter which permits simplification of its composition by providing a coil spring for restoring the pinion backward or a stopper for restricting its forward movement, and further can achieve coaxial and compact design as well.
Furthermore, it is still another object to provide a DC motor which can deter segments from bursting out even under a large force applied on the commutator due to the centrifugal force.